Strange clipping on blender donut
Problem: You probably wanted to make a drop shape for the icing. The Snapping feature is activated, and this pulled some vertices to a random spot somewhere. Now you have three faces on the bottom of your icing that go from one side to the other.
Snapping is the magnet icon. When it's activated the vertices snap to the mesh that is near. Sometimes it can happen that it attaches itself to a random spot of the donut somewhere. That's because &quot;near&quot; is, for example, x-z wise in the front view but not y-wise (to the back). So it's pulled back along the y-axis while x and z positions are only adjusted slightly.

It's easy to fix:

turn off the Snapping feature
select the icing object in Object mode and isolate it by pressing Numpad / (or menu: View &gt; Local View &gt; Toggle Local View). Then you will see only the icing. Inspect the bottom. 
go the Edit mode (Tab). (Most of your icing mesh is hidden, reveal it with Alt+H (or menu: Mesh &gt; Show/Hide &gt; Reveal Hidden))
select the renegade vertices and move them into place.
switch to Object mode (Tab) and leave Local view (Numpad /) to see the result